Mobile communication networks require a high degree of synchronization between a User Equipment (UE) and a base station (BS) in order to in order to effectively support wireless transmission and reception. Network configurations that use orthogonal frequency division multiplexing (OFDM) schemes for wireless transmission, such as Long Term Evolution (LTE), are particularly sensitive to synchronization mismatch. Accordingly, it is imperative to establish and maintain proper synchronization of wireless communications between a UE and an Evolved NodeB (eNodeB) in an LTE network.
Current LTE networks utilize OFDM for downlink transmissions, and consequently the reception of wireless signals by a UE must be closely synchronized with the corresponding transmission of the wireless signals by a cell located at a nearby eNodeB. Both timing synchronization and carrier frequency synchronization are vital to effective wireless communication.
Due to the potential the asynchronous transmission timing schedules between cells in LTE networks and signal propagation delays from different cells, a UE attempting to receive downlink signals from an cell must obtain timing synchronization. This may be conventionally performed using synchronization sequences such as Primary Synchronization Signals (PSSs) and Secondary Synchronization Signals (SSSs) that are periodically transmitted by a transmitting cell at an eNodeB. In addition to containing important information detailing the identity of a transmitting cell, these PSSs and SSSs may be used to align the downlink reception schedule of a UE with the corresponding downlink transmission schedule from a particular cell. A UE may subsequently receive downlink transmissions and perform uplink transmissions to and from a cell using the newly synchronized timing schedule. Due to the sensitive nature of OFDM-based systems, small inaccuracies in timing synchronization may result in significant performance degradation. Acquisition of accurate timing synchronization information is thus an essential element to effective network operation.
Carrier frequency synchronization is similarly vital to LTE network performance. Carrier frequency synchronization may be necessary due imperfections in receiver and transmitter oscillators as well as Doppler shifts. These oscillator imperfections may result in a carrier frequency offset, in which the carrier frequency used by a receiver to demodulate a wirelessly modulated signal does not match the carrier frequency used by the transmitter. The resulting carrier frequency offset may be addressed by identifying a carrier frequency offset value and performing appropriate compensations on the receiver side.
Utilizing a high quality local oscillator in a UE for carrier frequency generation may reduce the degree of carrier frequency offset, thereby improving synchronization. However, component ageing as well as the use of poor precision quality parts for cost reduction purposes may result in local oscillator deviations upwards of 20 parts per million (ppm) from a target value. Additionally, new applications of LTE communications in a variety of machine devices from pacemakers to car keys (e.g. for Internet of Things (IoT)) has resulted in the adoption of standards (e.g. by the 3rd Generation Partnership Project, or 3GPP) that allow for the use of low-cost oscillators with accuracy of +/−10 ppm. These relatively high local oscillator inaccuracies therefore require a robust timing and frequency synchronization scheme for LTE receiver devices that are capable of tolerating a large frequency offset.
The LTE standard has adopted the use of Zadoff-Chu sequences as the aforementioned PSSs used for UE to cell synchronization. While utilization of these sequences offers a variety of benefits such as reduced interference between cells, the sequences themselves are also uniquely susceptible to carrier frequency offset. High levels of carrier frequency offset may significantly affect performance (due to e.g. damage of the autocorrelation property of Zadoff-Chu sequences), and accordingly it is imperative that carrier frequency offset be appropriately compensated for.
A conventional method of identifying and subsequently addressing carrier frequency offset involves identifying several different carrier frequency offset hypotheses in a presumed range of possible carrier frequency offset values. Cell search and measurement is performed following by Physical Broadcast Channel (PBCH) decoding. Timing and carrier frequency offset values may then be determined based on the results, and subsequently used for synchronization purposes.
However, each hypothesis test can take up to several hundred milliseconds to complete PBCH decoding, and accordingly this process suffers from long latency. This long latency badly deteriorates the user experience for a variety of scenarios, such as e.g. a UE waking up from sleep state. Consequently, a simultaneously low-cost and low-latency approach is desired to obtain proper synchronization information.